Speaking on ABC News on Sunday night, Green said he felt safe calling Saturday's by-election, slightly more than 24 hours after the polls closed.

"He is a certain winner," Green said.

The race for the seat, which had been vacated by disgraced Liberal MP Daryl Maguire, became a two-man battle between Labor's Dan Hayes and Dr McGirr after the Liberals were hit with a 29 per cent swing against them.

"I don't think any one of us could have foreseen the circumstances that occurred during the campaign," Ms Berejiklian said.

"The people of the Wagga region sent us a strong message.

"I regret that the people of the Wagga region came to the conclusion that they could no longer trust the way in which they were being represented.

"They were angry and disappointed at the actions of the former member, and I share that."

The Liberal Party has held the safe Riverina seat for about six decades since winning it in a by-election in 1957, before disgraced MP Daryl Maguire resigned after seeking payment over a property deal.

"There's an underlying distrust of how Government works," Ms Berejiklian said.

"And I need to restore that and demonstrate to the community that we're here for them."

Federal lib spill factored in, says Premier

"When you put over the top of that, what occurred at a different level of government, that certainly exacerbated what the community was already feeling, and I want to say to the community — I've heard what you had to say.

"My Government has heard what you had to say, and I also appreciate that we will redouble our efforts to deliver for you into the future," she said.

"It was very interesting in this election. People weren't raising a lot of local issues or policy issues.

"This was really about trust in the political system, trust in politicians. And the by-election result exacerbated that. It was not an election where people were worried about a particular local issue."

Nats could have won, says party member

Wagga-based Nationals MLC Wes Fang said he believed his party could have won if they ran a candidate.

Mr Fang was critical of the Liberal campaign and said the Coalition will have to work hard to win back the trust of voters.

"They've run a city strategy in a country seat and it hasn't played well, that's pretty obvious from the result today (Sunday)," he said.

"It would have been good for us to highlight the wins that we've actually received for the electorate, and there's been many, we just don't communicate those wins very often."